Cells important in tissue homeostasis and repair, and in suppression of alloreactivity. (A) MSC, ILC, MDSC, iNKT, and Treg have all been shown effective in the prevention of acute GVHD when adoptively transferred. Early acute GVHD could in addition be successfully treated with the adoptive transfer of ILC2. (B) Cytotoxic T cells (Tc) induce apoptosis of MSC in the lungs, which leads to suppression of adaptive lymphocytes via IDO produced by phagocytosing cells. Of all cell products, MSC are the only ones that are frequently used in human GVHD. IDO, indoleamine 2,3-dioxygenase.